Boron crosslinkers for fracturing fluids with appreciably lower polymer loading

ABSTRACT

Fracturing fluid compositions and methods of fracturing subterranean formations using polyboronic compounds as crosslinking agents are provided. The compositions and methods of the present invention allow for lower polymer loadings because achieving higher fracturing fluid viscosities can be achieved using less polymer than in traditional crosslinked systems.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to compositions and methods of fracturinghydrocarbon producing formations. More specifically, the presentinvention relates to a crosslinking system for use with the fracturingfluids to increase viscosity of the fracturing fluids.

2. Description of the Related Art

Hydraulic fracturing techniques are widely used to enhance oil and gasproduction from subterranean formations. During hydraulic fracturing, afluid is injected into a well bore under high pressure. Once the naturalreservoir fracture gradient is exceeded, the fracturing fluid initiatesa fracture in the formation that generally continues to grow duringpumping. The treatment design generally requires the fluid to reach amaximum viscosity as it enters the fracture that affects the fracturelength and width. The viscosity of most fracturing fluids is generatedfrom water-soluble polysaccharides, such as galactomannans or cellulosederivatives. Linear gels that can be operated at ambient temperature donot have the necessary viscosity for proper proppant transferring atelevated temperature. The use of crosslinking agents or crosslinkers,such as borate, titanate, or zirconium (Zr) ions, can further increasethe viscosity. The gelled fluid can be accompanied by a propping agent(i.e., proppant) that results in placement of the proppant within thefracture that has been produced. The proppant remains in the producedfracture to prevent the complete closure of the fracture and to form aconductive channel extending from the well bore into the formation beingtreated once the fracturing fluid is recovered.

Guar based fracturing fluids are the most commonly used fluids inreservoir stimulation. As indicated previously, stimulation of oil andgas wells has been improved by the ability to crosslink fracturingfluids, such as guar. Crosslinking agents are used to significantlyimprove the viscosity of the system for various downhole conditions.Some common crosslinking agents include boron and zirconium or othermetallic compounds. Boron crosslinked gels are more commonly used due toits reversibility to mechanical shearing and favorable environmentalproperties.

While boron and zirconium crosslinking agents are effective for manytypes of guar based fracturing fluids, a certain amount of the guarpolymer is needed to achieve the viscosity necessary to fractionate theformation. It is desirable to use as little polymer as possible in afracturing fluid so that the overall cost of the fracturing job islower, less polymer residue remains in the fracture and the sand packafter breaking, and formation damage is minimized.

In view of the foregoing, a need exists for a crosslinking agent thatwould effectively increase the viscosity of the polymer, whichsimultaneously reduces the polymer loading as much as possible infracturing fluids. Additionally, it would be advantageous if suchcrosslinking system is compatible with existing fracturing systems.

SUMMARY OF THE INVENTION

In view of the foregoing, crosslinked fracturing fluids and methods offracturing subterranean formations are provided as embodiments of thepresent invention. The compositions and methods described herein areeffective and allow for lower polymer loadings in fracturing jobs.

As an embodiment of the present invention, a fracturing fluidcomposition is provided. In this embodiment, the fracturing fluidincludes a hydratable polymer capable of gelling in the presence of acrosslinking agent comprising a polyboronic compound.

Besides the compositional embodiments, methods of fracturingsubterranean formations are also provided as embodiments of the presentinvention. For example, as another embodiment of the present invention,a method of fracturing a subterranean formation is provided. In thisembodiment, water and a hydratable polymer capable of gelling areblended together and allowed to hydrate to form a hydrated polymersolution. Once the hydrated polymer solution is formed, a crosslinkingagent comprising a polyboronic compound is added to the hydrated polymersolution to produce a crosslinked fracturing fluid. The crosslinkedfracturing fluid is then injected into the subterranean formation tofracture the formation.

As another example, a method of fracturing a subterranean formation isprovided as an embodiment of the present invention. In this embodiment,a fracturing fluid comprising a hydratable polymer is crosslinked bycontacting the fracturing fluid with a polyboronic compound to produce acrosslinked fracturing fluid. The crosslinked fracturing fluid of thepresent invention has a higher viscosity when compared with thefracturing fluid being crosslinked with a conventional boric acidcompound as a crosslinking agent at the same polymer loading. Thecrosslinked fracturing fluid is then injected into the subterraneanformation to fracture the formation.

The resulting viscosity of the fracturing fluid of the present inventionis higher than the resulting viscosity of fracturing fluids of the samepolymer loading using conventional boric acid as the crosslinking agent.The increased viscosity of the crosslinked fracturing fluid of thepresent invention allows for a less amount of polymer to be used toachieve comparable results as prior art crosslinked fracturing fluidshaving higher polymer loadings. The resulting fracturing fluid of thepresent invention has a lower Ccc than the same polymer beingcrosslinked with conventional boric acid crosslinking agent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart showing the viscosity (cP) of GW-3 guar at variousconcentrations (ppt) using various crosslinking agents in accordancewith embodiments of the present invention and in accordance with priorart embodiments.

FIG. 2 is a chart showing the viscosity (cP) of GW-45 guar derivative atvarious concentrations (ppt) using various crosslinking agents inaccordance with embodiments of the present invention and in accordancewith prior art embodiments.

While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. However,it should be understood that the invention is not intended to be limitedto the particular forms disclosed. Rather, the intention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Illustrative embodiments of the invention are described below as theymight be employed in the operation and in the treatment of oilfieldapplications. In the interest of clarity, not all features of an actualimplementation are described in this specification. It will of course beappreciated that in the development of any such actual embodiment,numerous implementation-specific decisions must be made to achieve thedevelopers' specific goals, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure. Further aspects and advantages of the variousembodiments of the invention will become apparent from consideration ofthe following description.

As an embodiment of the present invention, a crosslinked fracturingfluid composition is provided. In this embodiment, the fracturing fluidincludes a hydratable polymer capable of gelling in the presence of acrosslinking agent comprising a polyboronic compound. Typical hydratablepolymers include, not limited to, polysaccharide, guar gum, guar gumderivatives, locust bean gum, karaya gum, carboxymethyl cellulose,carboxymethylhydroxyethyl cellulose, hydroxyethyl cellulose, orcombinations thereof. Various types of polyboronic compounds can be usedin embodiments of the present invention, as described herein.Conventional boron crosslinking agents used in hydraulic fracturingfluids are generally composed of borate salts or esters. The polyboroniccompounds of the present invention are more effective when compared tothe conventional boron crosslinking agents, which enables users to lowerthe polymer loading for fracturing jobs.

Besides the compositional embodiments, methods of fracturingsubterranean formations are also provided as embodiments of the presentinvention. For example, as another embodiment of the present invention,a method of fracturing a subterranean formation is provided. In thisembodiment, water and a hydratable polymer capable of gelling in thepresence of a crosslinking agent are blended together and allowed tohydrate to form a hydrated polymer solution. Once the hydrated polymersolution is formed, a crosslinking agent comprising a polyboroniccompound is added to the hydrated polymer solution to produce acrosslinked fracturing fluid. The crosslinked fracturing fluid is theninjected into the subterranean formation to fracture the formation.

As another example, a method of fracturing a subterranean formation isprovided as an embodiment of the present invention. In this embodiment,a fracturing fluid comprising a hydratable polymer is crosslinked bycontacting the fracturing fluid with a polyboronic compound to produce acrosslinked fracturing fluid. The crosslinked fracturing fluid of thepresent invention has a higher viscosity when compared with thefracturing fluid being crosslinked with conventional boric acidcrosslinking agents. The crosslinked fracturing fluid is then injectedinto the subterranean formation to fracture the formation.

The amounts of the components within the fracturing fluid can be variedin various embodiments of the present invention. For example, thepolyboronic compound can be present in a range of about 0.02 vol. % toabout 0.5 vol. % of the fracturing fluid composition; alternatively, ina range of about 0.10 vol. % to about 0.25 vol. %. In an aspect, thepolyboronic compounds can be present in a range that is effective forachieving the desired viscosity of the resulting fracturing fluid, aswill be apparent to those of skill in the art.

The methods and compositions described herein can be used with varioustypes of fracturing fluid systems. The hydratable polymer can be varieddepending upon the needs of a particular fracturing job. For example,the hydratable polymer can be guar gum, guar gum derivatives, locustbean gum, karaya gum, carboxymethyl cellulose, carboxymethylhydroxyethylcellulose, hydroxyethyl cellulose, or combinations thereof. Othersuitable hydratable polymers that are compatible with the methods andcompositions described herein can be used and are to be consideredwithin the scope of the present invention.

The methods and the compositions described herein are very efficient andhave a lower polymer loading when compared with the same polymer systembeing crosslinked using conventional crosslinking agents, such as boricacid. The methods and compositions described herein can have a higherviscosity when compared with the same amount of polymer that has beencrosslinked with conventional crosslinking agents, such as boric acid.In an aspect, the fracturing fluid composition of the present inventionhas a Ccc of less than about 12 ppt. In another aspect, the fracturingfluid composition of the present invention has a Ccc less than about15.5 ppt.

In an aspect, various compounds can be used as the polyboronic compoundused in embodiments of the present invention. Suitable polyboroniccompounds can include 2,5-thiophenediboronic acid (TDBA),1,4-benzenediboronic acid (BDBA), 4,4′-biphenyldiboronic acid (BPDBA),or combinations thereof. In an aspect, the polyboronic compounds caninclude compounds having the following structures:

or combinations thereof, wherein, R₁-R₈ can be hydrogen, alkyl group,alkenyl group, alkynyl group, aryl group, or combinations thereof. R₁-R₈can be, but is not required to be, identical and they can also be fromthe same fragment to form ring structures (such as, R₁, R₂═—CH₂CH₂—,—C(CH₃)₂C(CH₃)₂—, etc); X can be carbon, nitrogen, silicon, orcombinations thereof. In an aspect, a compound with X being nitrogen toincorporate multiple boron atoms into the structure by the chemicalbonding between N and B atoms is acceptable. Y can be a spacer, whichcan be straight chain of —(CH₂)—, straight chain with pendant(s),straight chain with branching, aromatic ring(s) directly connected,aromatic ring(s) indirectly connected, fused aromatic rings,heterocyclic ring(s) directly connected, heterocyclic ring(s) indirectlyconnected, fused heterocyclic rings, aliphatic ring(s) directlyconnected, aliphatic ring(s) indirectly connected, fused aliphaticrings, or combinations thereof. For example, Y can be phenylene,biphenylene, triphenylene, fluorene, fluorenone, naphthalene, methylenebisphenylene, stilbene, or combinations thereof. In an aspect, X canalso be part of Y when Y has ring structure(s). Z can be carbon,silicon, oxygen, nitrogen, alkyl group, alkenyl group, alkynyl group,aromatic ring(s), aliphatic ring(s), heterocyclic ring(s), orcombinations thereof. Z can also be a metal atom, such as, Al, Zr, Ti,Zn, or the like connected to other parts of the structure via chelationand/or other chemical interactions. Z can also be a fragment of Y. Thegeneral structure of suitable polyboronic compounds can be furtherextended to dendrimeric “poly” boronic compounds. Other suitable typesof polyboronic compounds will be understood by those of skill in the artand are to be considered within the scope of the present invention.

The polyboronic compounds belong to a different type of chemistry fromconventional boric acid, its ester derivatives and polyboric acids andtheir salts. In the chemistry nature of these boric acids or theirderivatives, boron atoms are not connected to any atom other thanoxygen, which leads to hydrolyzation in aqueous solution and release ofboric acid. When used as crosslinking agents, the actual crosslinkingspecies is boric acid after hydrolysis of borate esters or polyborates.When boron atom is connected to at least one atom other than the oxygenatoms, especially carbon or nitrogen, the corresponding compounds arecalled boronic acids (or boronic esters) and they are differentcompounds and possess different chemical properties. When they contactwater or base, even at elevated temperatures, the B—C (or B—N, B—Si,etc.) bond will not hydrolyze, and therefore the active crosslinkingspecies is not boric acid, but polyboronic compounds instead. When thesediboronic or polyboronic compounds are used as crosslinking agents, theywill provide two or more boron atom sites, each is capable of beingchelated with the two cis-hydroxyls in the backbone of the hydratablepolymers. Therefore, a triboronic compound can crosslink threepolysaccharide chains via three boron atoms within one crosslinkingagent molecule. In other words, the polyboronic species are thecrosslinking agents, not boric acid hydrolyzed from correspondingesters, as shown in prior art, as delayed boric acid crosslinkingagents. In an aspect, when N is attached to B, to form stable(OR)₂BNHRHNB(OR)₂ structure is particularly preferred.

Besides the polymer and crosslinking agents described herein, variousadditives can be useful in the present invention. Additives used in theoil and gas industry and known in the art, including but not limited to,corrosion inhibitors, non-emulsifiers, iron control agents, delayadditives, silt suspenders, flowback additives, pH adjusting agents,clay stabilizer, surfactants, and gel breakers, can also be used inembodiments of the present invention. Proppants including, but notlimited to, frac sand, resin coated sand, quartz sand grains, ceramicproppant, tempered glass beads, rounded walnut shell fragments, aluminumpellets, and nylon pellets at desired size can also be used. Proppant istypically used in concentrations that range between about 1 pound pergallon of the fracturing fluid composition to about 8 pounds per gallonof the fracturing fluid composition. Other suitable additives useful inthe present invention will be apparent to those of skill in the art andare to be considered within the scope of the present invention.

The fracturing fluid of the present invention can be used by pumping thefluid into a well bore penetrating the subterranean formation to befractured. The fracturing fluid is injected at a rate sufficient tofracture the formation and to place proppant into the fracture.

As another advantage of the present invention, lower loadings of polymercan be used to obtain equivalent fracturing fluid performance at reducedoverall treatment costs. Reduced polymer loadings can also result inless damage to the surrounding subterranean formation after thefracturing treatment. Guar based polymers are attributed with causingdamage to the fracture sand pack and reducing the effective fracturewidth. The present invention permits substantial reduction in the amountof polymer injected into the formation while maintaining optimal fluidproperties for creating the fracture.

C*, C**, and Ccc are often used as the leading indexes to represent theefficiency of a crosslinked polymer fluid. C*, C** and Ccc depend on thetype of polymer being used, as well as, possibly the type ofcrosslinking agent used. As used herein, the term “Ccc” is used todescribe the critical crosslinking concentration for polymer chains, aswill be understood by those of skill in the art. The term “Ccc” isgenerally considered to be minimum polymer concentration where the fluidis able to be crosslinked It was proposed by others that Ccc is largelyindependent on the type of crosslinking agent used while depends on onlythe type of polymer that was used. As a result of the findings relatedto the present invention, it was discovered that the conventional theoryis not necessarily true. The examples described herein show that thetype of crosslinking agent used can affect Ccc, which is contrary towhat was previously believed. The structures of the crosslinking agentsof the present invention lowered the Ccc of guar polymer sosignificantly that the polymer solutions can be effectively crosslinkedat concentrations much lower than widely accepted Ccc values.

EXAMPLES

The following examples are included to demonstrate the use ofcompositions in accordance with embodiments of the present invention. Itshould be appreciated by those of skill in the art that the techniquesdisclosed in the examples that follow represent techniques discovered bythe inventors to function well in the practice of the invention.However, those of skill in the art should, in light of the presentdisclosure, appreciate that many changes can be made in the specificembodiments that are disclosed and still obtain a like or similar resultwithout departing from the scope of the invention.

Example 1

Example 1 was used to determine Ccc and to test the effectiveness of thecrosslinking agents made in accordance with embodiments of the presentinvention. 25 ppt (0.3%) solution of guar gum (GW-3, which iscommercially available from BJ Services Company) was prepared byhydrating GW-3 powder. After at least 30 minutes, the solution wassystematically diluted to obtain 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,17, 18, 19, 20, 21, 22, 23, 24 ppt solutions. Other additives (such asbuffer, clay stabilizer, and bactericide) were added to the GW-3 guarsolution. The crosslinking agents made in accordance with embodiments ofthe present invention were then added by mixing. Thepolymer/crosslinking agent ratio was kept constant. The viscosity of thecrosslinked gel was measured on Fann 35 instrument at room temperature.The polymer/crosslinking agent ratios were as follows:

TDBA/GW-3 = 0.079 BDBA/GW-3 = 0.039 BPDBA/GW-3 = 0.056 BA/GW-3 = 0.022TDBA/GW-45 = 1.10 BDBA/GW-45 = 1.13 BA/GW-45 = 0.2233

The ratios were kept constant within each crosslinking agent to obtainsystematic readings. The viscosity was plotted against the concentrationto observe changes in viscosity versus concentration change, as shown inFIGS. 1 and 2. As concentration increases, a change in slope occurs. Theinterception of the slopes of the two regions defines Ccc.

Example 2

The Ccc values were calculated for two different types of guarfracturing fluids, GW-3 and GW-45, that were crosslinked with fourdifferent types of crosslinking agents. GW-3 and GW-45 are guar basedpolymers commercially available from BJ Services Company. As can be seenin Table 1, a lower Ccc was obtained using the various polyboroniccompounds (i.e., TDBA, BDBA, and BPDBA) when compared to the same guarpolymers being crosslinked with conventional boric acid (BA). Theresults of this example show that the type of crosslinking agent cangreatly affect the Ccc, which is contrary to what is conventionallyaccepted in the industry.

TABLE 1 C_(cc), ppt Polymer BA TDBA BDBA BPDBA GW-3 15 9 8.5 8 GW-4515.5 12 8.5

Example 3

In this example, the viscoelastic properties (n′) and viscosities (cP)of two crosslinked guar polymer systems were compared. 15 ppt of GW-3was crosslinked with 0.27 mmol BPDBA at 150° F. and compared with atypical crosslinked system that was prepared by crosslinking 20 ppt ofGW-3 with CXB-10 (Lightning 2000), which is commercially available fromBJ Services Company. As shown in Table 2, the results clearlydemonstrate that these polyboronic compounds used in embodiments of thepresent invention can effectively lower polymer loading for fracturingstimulation. The reduction of the polymer loading is related to the sizeof the group separating the two boronic acids.

TABLE 2 15 ppt GW-3, 0.27 mmol BPDBA Lighting 2000 Viscosity (cP) atViscosity (cP) at Time, min n′ 40 sec⁻¹ 100 sec⁻¹ 170 sec⁻¹ n′ 40 sec⁻¹100 sec⁻¹ 511 sec⁻¹ 2.1 0.671 481 356 299 0.389 1855 1060 391 32.1 0348582 320 227 0.533 349 228 106 62.1 0.358 665 369 263 0.871 343 305 24792.1 0.276 694 358 244 0.753 317 253 169 122.1 0.443 489 293 218 0.814314 265 195

Example 4

Example 4 illustrates one embodiment of the synthetic preparation of apolyboronic compound having the following structure. This example can beused to illustrate how polyboronic compounds are generally synthesized.The synthesis scheme can be extended to other type of polyboroniccompounds.

In this example, a 150 mL 3-necked round bottom flask was equipped witha reflux condenser guarded with a CaCl₂ drying tube, a temperatureindicator and a pressure-equalizing addition funnel. Into the flask wasadded 7.3 g tris(2-aminoethyl)amine, followed with 15 g anhydrous MeOH.Under nitrogen, 20.8 g freshly distilled trimethyl borate wastransferred into the addition funnel and was then diluted with 6.9 ganhydrous MeOH. Under magnetic agitation, the trimethyl borate solutionwas added drop by drop into the flask at a temperature below 40° C.After the completion of the addition, the resultant solution was allowedto stand at room temperature for 30 minutes and then heated to refluxfor at least 4 hours. The resulting compound can be used as acrosslinking agent in accordance with embodiments of the presentinvention, such as those described in Example 5.

Example 5

In Example 5, the viscoelastic properties (n′) and viscosities (cP) oftwo crosslinked guar polymer systems were compared. 25 ppt of GW-3 wascrosslinked with 0.4 mmol compound prepared in Example 4 and 4 gpt 25%sodium hydroxide (NaOH) at 150° F. and compared with an optimizedcrosslinked system that was prepared by crosslinking 25 ppt of GW-3 withCXB-10 (Lightning 2500), which is commercially available from BJServices Company. As shown in Table 3, the results clearly demonstratethat these polyboronic compounds used in embodiments of the presentinvention can effectively lower polymer loading for fracturingstimulation.

TABLE 3 25 ppt GW-3, 0.4 mmol XLB, 4 gpt 25% NaOH Lightning 2500Viscosity (cP) at Viscosity (cP) at Time, min n′ 40 sec⁻¹ 100 sec⁻¹ 170sec⁻¹ n′ 40 sec⁻¹ 100 sec⁻¹ 170 sec⁻¹ 2.1 0.4827 1091 568 389 0.37833924 2220 1596 32.1 0.3170 1648 923 660 0.2807 1354 700 478 62.1 0.40611646 1047 806 0.3011 1332 702 485 92.1 0.3914 1433 918 709 0.3098 1453772 535 122.1 0.4074 1335 824 623 0.4278 1355 802 592 152.1 0.5785 1264883 717 0.3857 1166 664 479 182.1 0.6394 1169 872 736 0.1443 1337 610388

All of the compositions and/or methods disclosed and claimed herein canbe made and executed without undue experimentation in light of thepresent disclosure. While the compositions and methods of this inventionhave been described in terms of preferred embodiments, it will beapparent to those of skill in the art that variations can be applied tothe compositions and/or methods and in the steps or in the sequence ofsteps of the methods described herein without departing from theconcept, spirit and scope of the invention. More specifically, it willbe apparent that certain agents that are chemically related can besubstituted for the agents described herein while the same or similarresults would be achieved. All such similar substitutes andmodifications apparent to those skilled in the art are deemed to bewithin the scope and concept of the invention.

1. A method of fracturing a subterranean formation comprising the stepsof: a. blending together water and a hydratable polymer capable ofgelling in the presence of a crosslinking agent; b. allowing thehydratable polymer to hydrate to form a hydrated polymer solution; c.adding a crosslinking agent comprising a polyboronic compound to thehydrated polymer solution to produce a crosslinked fracturing fluid,wherein the polyboronic compound is of the formula:

wherein R₁-R₆ is hydrogen, alkyl group, alkenyl group, alkynyl group,aryl group, or combinations thereof; X is carbon, nitrogen, silicon, orcombinations thereof; Y is a straight chain of —(CH₂)—, a straight chainwith pendant(s), a straight chain with branching, aromatic ring(s)directly connected, aromatic ring(s) indirectly connected, fusedaromatic rings, heterocyclic ring(s) directly connected, heterocyclicring(s) indirectly connected, fused heterocyclic rings, aliphaticring(s) directly connected, aliphatic ring(s) indirectly connected,fused aliphatic rings, or combinations thereof; and Z is carbon,silicon, oxygen, nitrogen, alkyl group, alkenyl group, alkynyl group,aromatic ring(s), aliphatic ring(s), heterocyclic ring(s), a metal atom,or combinations thereof; and d. injecting the crosslinked fracturingfluid into the subterranean formation to fracture the formation.
 2. Themethod of claim 1, wherein: R₁-R₆ are from a same fragment to form aring structure; Y is phenylene, biphenylene, triphenylene, fluorene,fluorenone, naphthalene, methylene bisphenylene, stilbene, orcombinations thereof; X is part of Y when Y has ring structure(s); Z isAl, Zr, Ti, Zn, or combinations thereof; Z is a fragment of Y; orcombinations thereof.
 3. The method of claim 1, wherein the polyboroniccompound is present in a range of about 0.02 vol. % to about 0.5 vol. %of the fracturing fluid composition.
 4. A method of fracturing asubterranean formation comprising the steps of: a. crosslinking afracturing fluid comprising a hydratable polymer by contacting thefracturing fluid with a polyboronic compound to produce a crosslinkedfracturing fluid, wherein the polyboronic compound is of the formula:

wherein R₁-R₆ is hydrogen, alkyl group, alkenyl group, alkynyl group,aryl group, orcombinations thereof; X is carbon, nitrogen, silicon, orcombinations thereof; Y is a straight chain of —(CH₂)—, a straight chainwith pendant(s), a straight chain with branching, aromatic ring(s)directly connected, aromatic ring(s) indirectly connected, fusedaromatic rings, heterocyclic ring(s) directly connected, heterocyclicring(s) indirectly connected, fused heterocyclic rings, aliphaticring(s) directly connected, aliphatic ring(s) indirectly connected,fused aliphatic rings, or combinations thereof; and Z is carbon,silicon, oxygen, nitrogen, alkyl group, alkenyl group, alkynyl group,aromatic ring(s), aliphatic ring(s), heterocyclic ring(s), a metal atom,or combinations thereof; and b. injecting the crosslinked fracturingfluid into the subterranean formation to fracture the formation.
 5. Themethod of claim 4, wherein: R₁-R₆ are from a same fragment to form aring structure; Y is phenylene, biphenylene, triphenylene, fluorene,fluorenone, naphthalene, methylene bisphenylene, stilbene, orcombinations thereof; X is part of Y when Y has ring structure(s); Z isAl, Zr, Ti, Zn, or combinations thereof; Z is a fragment of Y; orcombinations thereof.
 6. The method of claim 4, wherein the polyboroniccompound is present in a range of about 0.02 vol. % to about 0.5 vol. %of the fracturing fluid composition.